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She has short blue hair with long bangs that will cover half her face at times. Her eyes are much rounder than that of an average person and has a few freckles on her nose. Because of the circumstances of her birth, Isma can change between human and merrow forms by saying her true name.

Though human, Isma is born the child of a human fisherman and a female Merrow. Because of her half-breed lineage, and her father forced to bide by the villagers' taboo on Merrow for sealing the Sea God that they have come to worship, Isma was sent to live in the outskirts of the village on the island. It was only when her father was on the verge of death, getting some hints from bullying children, that Isma learns her mother is a merrow while her father gave her a charm that wards off evil while promising her that her mother will come for her.

After the World Transformation weakened the seal keeping the Sea God in check, Isma's charm allowed her to be the sole survivor as everyone else on the island had been eaten by the creature and became its tentacle extensions. When the Sea Horse arrived to make repairs, Isidro and Puck meet Isma before they are forced by Schierke as she sensed Isma's Merrow lineage. After nightfall and the initial attack by the Sea God's tentacles, Isma is allowed to come aboard the Sea Horse while Guts goes after the Sea God before it finally broke free while destroying its island prison. During the chaos that ensued, hearing the voices of the Merrow at the time, Isma dives after Isidro when he fell overboard and saves him by speaking her true name. After being warned by Ivalera to not divulge her true name, Isma aids the Merrow in fighting the Sea God and is reunited with her mother in the process. Following the demise of the Sea God, Isma reverted to her human form out of shyness of swimming with her maternal family and remains on the Sea Horse as she accompanies the crew to Skellig.